Collaborations and Hierarchies CS 4311 Chapters 5 and 6 of Wirfs-Brock, R., Wilkerson, B., and Wiener, L., Designing Object-Oriented Software, Prentice Hall, 1990. CS 4311

Outline Collaborations Hierarchies Identifying collaborations Recording collaborations Hierarchies Hierarchy graphs Venn diagrams CS 4311

Motivation for Collaborations Two ways to perform responsibilities ? Collaboration is Request from one object to another in order to fulfill a responsibility. CS 4311

Motivation (Cont.) Why identify collaborations? Collaborations represents flow of control and information through the system, May identify misplaced responsibilities, and May identify missing responsibilities. In sum, shows dynamics of the system. CS 4311

Finding Collaborations Look at each responsibility of each class: Is the class capable of fulfilling this responsibility by itself? If not, where can it get what it needs? Look at each class: What other classes needs what this class does or knows? CS 4311

Finding More Collaborations Examine relationships between classes, especially: The “is-part-of” relationship, and The “has-knowledge-of” relationship. CS 4311

“Is-part-of” Relationship May imply responsibilities for maintaining information. May fulfill responsibilities by delegating them. Two of relationships: Composite Aggregate (or container) Q: Which relationship is more likely to require collaborations? CS 4311

“Has-knowledge-of” Relationship May know other classes that are not in part-of relationships (i.e., associations in UML). May imply responsibilities to know information, and thus collaborations. Q: Can you think of an example? CS 4311

Recording Collaborations Write the name of the server (or helper) class on the CRC card of the client. Write the name directly to the right of the responsibility the collaboration fulfills. Client Class: Person Responsibilities Collaborations Knows name Knows address AddressBook Knows phone number PhoneBook … Server (or helper) CS 4311

Collaboration Graph Graphical representation of collaborations Arrow from client to a “contract” of the server, denoted by a semicircle Contract: group of responsibilities (more on this later) AddressBook 1 Person PhoneBook 2 CS 4311

Example: Mail-Order System Class Responsibilities Collaborators Catalog Create a catalog Mail a catalog Order Process an order Track an order Product Ship a product Payment Supplier Customer CS 4311

Other Tools - UML UML interaction diagrams Message sequence diagram Communication diagram (a.k.a. collaboration diagram) CS 4311

Message Sequence Diagram object lifetime control message CS 4311

Message Sequence Diagram Collaboration CS 4311

Communication Diagram object link message CS 4311

Communication Diagram Collaboration CS 4311

Outline Collaborations Hierarchies Hierarchy graph Venn diagram CS 4311

Review on CRC Process Exploratory phase Analysis phase Identify a preliminary list of classes, responsibilities and collaborations. Analysis phase Obtain a more global understanding of the design, e.g., by using tools such as: Hierarchy graphs, Venn diagrams, and Contracts. CS 4311

Hierarchy Graph A graphical representation of inheritance between related classes. A hierarchy graph is a general tree. The nodes are classes and the arcs represent inheritance. Ancestor nodes are superclasses of descendent nodes, which are subclasses. CS 4311

Example - Notation of [Wirfs-Brock-etal90] PartTime FullTime Employee - Notation of [Wirfs-Brock-etal90] - Leaf nodes are often concrete classes. - Non-leaf nodes are often abstract classes. CS 4311

In UML … Employee {abstract} PartTime FullTime Employee (or ) CS 4311

Abstract Classes Classes that cannot be instantiated. Designed only to be inherited from, thus allowing reuse and avoiding duplication. Used to factor common behaviors among classes. CS 4311

Finding Abstract Classes At the exploratory stage, all identified classes are probably concrete and some may have been identified as abstract. But, do you have all your abstract classes? That is, have you used the power of abstraction (factoring behavior)? CS 4311

Another Example of Hierarchy Graph Ordered Collection Indexable Collection Array Magnitude Matrix String Date CS 4311

Venn Diagram Another tool to understand inheritance relationships. Views a class as a set of responsibilities. Thus, an intersection among classes: Denotes common responsibilities, and thus May indicates an abstract superclass. CS 4311

Example set of responsibilities PartTime Employee FullTime set of objects PartTime FullTime Employee set of responsibilities PartTime FullTime Employee PartTime FullTime Employee CS 4311

Exercise Draw a Venn diagram for the following hierarchy graph. Ordered Collection Indexable Collection Array Magnitude Matrix String Date CS 4311

In Sum, Hierarchies … Are for reviewing the inheritance relationships, and Use hierarchy graphs and Venn diagrams as a notation and an analysis tool CS 4311

Building Good Hierarchies Model “is-a” relationships. Factor common responsibilities as high as possible. Make sure abstract classes do not inherit from concrete classes. Eliminate classes that do not add functionality. CS 4311

Example – Using Venn Diagram To Check Hierarchy If B supports only a part of responsibilities defined for A, what does the Venn diagram look like? How to fix this? A B CS 4311

Fixing the Problem CS 4311

Example – Factoring Responsibilities Drawing Element Text Element Line Element Ellipse Element Rectangle Element Group Element CS 4311

Factoring (Cont.) CS 4311

Factoring (Cont.) CS 4311

Group Exercise Suppose that you are designing a new Java compiler and you identified the following concrete classes for various kinds of declarations allowed in Java. Class Declaration Interface Declaration Field Declaration Method Declaration Local Variable Declaration Method Parameter Declaration Organize the above classes into a class hierarchy. CS 4311